1. Technical Field
The present invention relates to a thin film solar cell and a manufacturing method thereof. More particularly, the present invention relates to a stacked-layered thin film solar cell whose light-absorbing layer has a p-i-n-type structure, and a method for making the same.
2. Description of Related Art
According to current thin film solar cell technology, thin film solar cells based on semiconductors layers of copper indium gallium diselenide (abbreviated as CIGS) are one of the most efficient solar cells of today. Formerly, the predecessor of the CIGS thin film solar cells consisted essentially of copper (Cu), indium (In), and selenium (Se) and is therefore known as the CIS (CuInSe2, copper indium selenide) thin film solar cell. It was not until gallium (Ga) or sulfur (S) was subsequently incorporated into the CIS thin film solar cells that the CIGS thin film solar cells, which deliver higher conversion efficiency than the CIS version, were produced. Nowadays, CIGS solar cells are mass-produced mainly by a vacuum-based process. In addition, a CIGS solar cell often requires a buffer layer, typically a cadmium sulfide (CdS) layer, because the operation of a CIGS solar cell depends chiefly on photoelectric conversion taking place at the heterojunction between the n-type cadmium sulfide layer and a p-type light-absorbing layer.
Furthermore, for a solar cell to have high photoelectric conversion efficiency, its band gap must be at least between 1.1 eV and 1.7 eV. In U.S. Pat. Nos. 5,078,804 and 5,141,564, a band gap ranging from 1.0 eV to 1.7 eV is achieved by adjusting the indium (In)/gallium (Ga) concentration ratios in the light-absorbing layer to different values.
Moreover, referring to FIG. 1, U.S. Pat. No. 5,981,868 teaches a thin film solar cell 1 which includes a glass substrate 2, a metallic back electrode layer 3, a light-absorbing layer 4, a light-absorbing surface layer 41, a buffer layer 5, a window layer 6, and an upper electrode layer 7. Light enters the thin film solar cell 1 in a direction indicated by S. The light-absorbing surface layer 41 is formed by sputtering gallium and indium onto a surface portion of the photoconductive layer 4 that is adjacent to the buffer layer 5 while adjusting the gallium/indium concentration ratio throughout the sputtering process. Thus, the light-absorbing surface layer 41 widens the band gap, and consequently the light absorption band, of the thin film solar cell 1, thereby enabling the thin film solar cell 1 to reach the optimal photoelectric conversion efficiency of 13.6%.
However, the method for making the foregoing CIGS thin film solar cell requires that the light-absorbing layer be formed in a vacuum environment, and the buffer layer in a general atmospheric environment; hence, the manufacturing process is rather complicated. More specifically, after the buffer layer is prepared, the semi-product must by transferred to a vacuum environment so as for the window layer to be deposited on the buffer layer. Apart from that, the junction between the buffer layer and the photoconductive layer is a heterojunction and therefore is highly prone to defects. Also, the cadmium sulfide (CdS) layer serving as the buffer layer contains cadmium (Cd), which is a poisonous substance and may lead to pollution during use. Besides, as gallium and indium are both rare elements, use of these elements in large quantity incurs high production costs.